The invention is directed to a process for the production of nearly isotropic spherically shaped fuel elements having increased requirements, particularly high heavy metal content for improved gas cooled high temperature reactors by pressing a molding powder consisting of a mixture of natural graphite and binder resin, synthetic graphite and binder resin or a mixture of both types of graphite powder and binder resin together with coated fissile and fertile fuel material particles.
Spherical fuel elements customarily consist of a fissile and fertile fuel material containing nucleus which is surrounded by a fuel free shell and is joined to it without transition (Hrovat, German OS No. 1,646,783).
The graphite matrix, i.e., the graphite material, is identical in the nucleus of the sphere and in the fuel free shell. The fuel element diameter in general is 40 to 80 mm, by preference about 60 mm and the thickness of the shell is 2 to 20 mm, by preference about 5 mm.
In the known spherical fuel elements the nucleus contains in homogeneous distribution the fissile and fertile fuel materials in the form of spherical heavy metal particles. To retain fission products the particles are provided with multiple coatings of pyrolytic carbon, in a given case with an intermediate layer of silicon carbide.
As fissile fuel material there is normally employed uranium 235 and as fertile material thorium 232, the fissile and fertile fuel material being employed as the carbide or oxide. While the fissile and fertile fuel materials in the so-called THTR element, the standard spherical element of the thorium high temperature reactor, are jointly present in the same particles, they are provided for in the so-called Feed-Breed-Element separated in discrete particles mixed with each other.
A series of requirements is placed on the spherical fuel elements:
They must have high strength properties with the least possible modulus of elasticity and small thermal coefficients of expansion. During the reactor operation, particularly at start up and shut down of the reactor proceed as a result of temperature gradients thermal stresses which can only be partially relaxed by creep processes and therefore produce heavy mechanical stresses in the fuel element spheres. Since in the charging of the reactor core and circulation of the sphere heap the fuel elements drop from several meters high to the sphere heap surface, there are high additional mechanical stresses. Additionally in the disconnecting of the reactor operation the absorber rods get into the sphere heap directly which leads to a further considerable load on the individual fuel elements. In order to guarantee a sufficiently high service life of the fuel elements there are required high values for compressive, bending and tensile strength of the fuel element matrix. For the previously mentioned reasons there are added the requirements of a good drop and abrasion resistance and particularly of a high crushing load of the spheres.
Besides they must have a high heat conductivity in order to hold the temperature gradients inside the sphere as small as possible.
Furthermore, a good corrosion resistance against trace impurities is necessary, as for example against water vapor, CO, CO.sub.2 and H.sub.2 which are contained in the helium cooling gas.
Besides there is an increased heavy metal content of the spherical fuel elements. In the so-called THTR-Standard-Fuel-Element the heavy metal content is 11 grams per sphere. To raise the conversion rate (formation of uranium 233 from thorium 232) a substantially higher heavy metal content of the fuel elements for advanced high temperature reactors is required. Thereby in spite of the increased heavy metal content in the production the requirements of extremely low fractions of defective coated particles in the molded spherical fuel element are intensified.
Besides a good irradiation behavior is necessary up to temperatures of about 1400.degree. C. and up to an exposure to fast neutrons (E&gt;0.1 MeV) of about 9.times.10.sup.21 neutrons/cm.sup.-2. This requirement assumes an as much as possible high crystallinity of the isotropic graphite matrix.
For the production of spherical fuel element previously, processes have been proposed in which first the lower half of the fuel free shell is formed in a metallic pressing die, then the fuel containing spherical nucleus inserted and subsequently the upper shell half pressed on (German Patent No. 1,096,513). Since the bulk density of the molding powder mixture is relatively small (about 0.5 g/m.sup.3) and merely is densified in the axial direction about four times the volume, in the pressing there cannot be avoided a preferred orientation of the customarily anisotropic constructed graphite starting particles. This has as a result an inadmissible anisotropy of the matrix of the sphere. In such a sphere there occur in the irradiation with fast neutrons high irradiation induced stresses which can lead to the formation of cracks and therewith endanger the mechanical integrity of the fuel element.
This disadvantage is avoided if in place of the die molding process with a steel tool there is used the semi-isostatic pressing in rubber molds of silicone rubber (Hrovat, German OS No. 1,646,783). The silicone rubber behaves in the pressing under pressure similarly to a liquid. Thereby there is attained an isotropic three-dimensional compressing of the molding powder. To take up the molding powder the rubber mold formed of two halves has a central, elliptic shaped cavity which is so proportioned that in the pressing there is formed a sphere having a diameter for example of about 60 mm. The prepared filled rubber mold is introduced into a steel die of the press and pressed together with the upper and lower punches. Because of the elastic behavior of the rubber there is used molding at room temperature and consequently a very high molding pressure is required. The fuel element spheres having a diameter of 60 mm are customarily compressed with a molding pressure of 3 metric tons/cm.sup.2 which at the required rubber mold size corresponds to a very high pressing force of 400 tons (i.e., 400 metric tons). Therewith so that at this high molding pressure no particles bordering each other are mutually damaged the particles are encased in molding powder. In order that the spheres produced from the encased particles maintain a sufficient strength according to Hrovat German Patent No. 1,909,871 only a part of the molding powder needed for the nucleus is used to encase the particles, the remaining part mixed with the encased particles and the mixture pressed to the nucleus. In this way there are produced fuel element spheres with isotropic properties with a limit of up to about 11 grams heavy metal content. At higher heavy metal contents of for example 20 to 30 grams per sphere, however, there cannot be avoided the destruction of a part of the coated particles in the pressing.
In German OS No. 2,246,163 (and related Rachor U.S. Pat. No. 3,912,798) to improve the course of the process there is proposed that the second pressing step in which the spherical nucleus embedded in a coating of graphite molding powder is pressed in a rubber mold is divided into two pressing stages wherein first there is carried out a preliminary pressing in a rubber mold at low pressure and then this preformed object is final pressed at high pressure. Here also at high metal content there cannot be avoided particle damage due to the high molding pressure.
Furthermore, there has been proposed a process according to which there is first produced from the binder resin containing graphite molding powder mixture a granulate having isometrically constructed particles of high bulk density and then hot pressing this granulate together with the coated fuel particles in the plastic range of the binder resin at the relatively very low pressure of 100-200 kp/cm.sup.2 to molded articles (German Patent No. 2,104,431 and related Hrovat U.S. Pat. No. 4,017,567). Indeed with this process there can be prepared prismatic molded articles with an extensive isotropic structure and high heavy metal content on which there is placed no requirements as to the drop strength and crushing load but no spherical fuel elements can be considered for the above mentioned requirements. The decisive reason for this is a relatively poor bond of the smooth surfaces of the individual granulate particles which are already precompressed. Therefore, this process is unsuited for the production of fuel element spheres with the required drop strength and crushing load.
The entire disclosures of the aforementioned German OS No. 1,646,783, German Patent No. 1,096,513, German Patent No. 1,909,871, U.S. Pat. No. 3,912,798 and U.S. Pat. No. 4,017,567 are hereby incorporated by reference and relied upon.